American Journal of Epidemiology Advance Access originally published online on October 8, 2007
American Journal of Epidemiology 2007 166(11):1356-1357; doi:10.1093/aje/kwm287
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LETTERS TO THE EDITOR |
TWO AUTHORS REPLY
National Center for Injury Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA 30341
(e-mail: LBeck{at}cdc.gov)
We would like to thank Dr. Marmor (1) for his comments on our paper (2). Our study used National Household Travel Survey (NHTS) data to estimate person-trips by six modes of travel, including walking. Although we agree with the author that the number of walking trips may be undercounted in the data, we disagree with his assessment of the magnitude of error and its effect on our conclusions.
The NHTS defines a trip as "travel from one address to another" (3, p. N-12). Thus, a single trip may occasionally include several segments that are taken by different modes. The most common circumstance in which this occurs involves the use of public transit, which represents 1.6 percent of all trips in the United States (4). For example, a person may walk to a bus stop, travel by bus, and finally walk from a bus stop to his/her ultimate destination. The NHTS counts this example as one trip, and the travel mode is recorded as bus.
Other factors may also influence estimates of traffic injury rates; however, it is first important to point out that our analysis concerns events that occur in traffic; that is, to be included in the Fatality Analysis Reporting System, a crash must involve a motor vehicle traveling on a trafficway customarily open to the public (5). The numerator data represent fatal and nonfatal injuries in traffic. Walking to a parked vehicle at a curb or parking lot typically does not involve travel on a public trafficway and thus should not be included in our count of walking trips. Walking to or from public transit is more likely to occur on a public trafficway, and the exclusion of such trips from our denominator would result in an overestimate of pedestrian injury rates. However, we do not believe that the inclusion of such trips would alter our finding that pedestrian fatality rates are higher than those for passenger-vehicle occupants.
The operationalization of traffic exposure may affect the estimates as well. Our analysis used person-trips as the unit of exposure to traffic. This measure assigned equal weight to all trips, regardless of their length. In the case of pedestrian trips, which tend to be shorter distances than motorized trips, a trip-based measure of exposure would underestimate pedestrian risk. We chose person-trips, rather than trip distance, as our exposure measure because of concerns about the validity of self-reported data on trip distance.
Although the above-mentioned factors could lead to over- or underestimates of rates, we stand by our conclusions, consistent with previous literature (6, 7), indicating that pedestrians are at increased risk for fatal traffic injuries, compared with passenger vehicle occupants.
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ACKNOWLEDGMENTS |
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The findings and conclusions in this letter are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.
Conflict of interest: none declared.
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References |
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 TOP References |
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- Marmor M. Re: "Motor vehicle crash injury rates by mode of travel, United States: using exposure-based methods to quantify differences." (Letter). Am J Epidemiol (2007) 166:1356.
[Free Full Text] - Beck LF, Dellinger AM, O'Neil ME. Motor vehicle crash injury rates by mode of travel, United States: using exposure-based methods to quantify differences. Am J Epidemiol (2007) 166:212–18.
[Abstract/Free Full Text] - Highway Federal Administration. 2001 National Household Travel Survey: user's guide. (2004) Washington, DC: US Department of Transportation.
- Highway Federal Administration. Summary of travel trends: 2001 National Household Travel Survey. (2004) Washington, DC: US Department of Transportation.
- National Highway Traffic Safety Administration. Traffic safety facts 2005: a compilation of motor vehicle crash data from the Fatality Analysis Reporting System and the General Estimates System. (2006) Washington, DC: US Department of Transportation. (Publication no. DOT HS 810 631).
- Pucher J, Dijkstra L. Promoting safe walking and cycling to improve public health: lessons from the Netherlands and Germany. Am J Public Health (2003) 93:1509–16.
[Abstract/Free Full Text] - Elvik R, Vaa T, eds. The handbook of road safety measures. (2004) Amsterdam, Netherlands: Elsevier.
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